Control devices of that kind are well known in the art for example, in U.S. Pat. No. 3,910,687 patent application No. 2,345,337. In these devices, a transparent container with two main surfaces facing a light source and a light-sensitive element, respectively, contains the ferromagnetic fluid and a transparent liquid immiscible with the ferromagnetic fluid, arranged so that under rest conditions the opaque ferromagnetic fluid may intercept the light rays sent towards the light-sensitive element; to let the light pass, a magnetic field is generated by electromagnets and causes the displacement of the ferromagnetic fluid which is replaced, in the region which is to be traversed by light rays, by the transparent liquid.
The disadvantage of these control devices is that they cannot assure the uniform illumination of light-sensitive material. These devices have the characteristics of the exposure time progressively decreasing from the ares in which the ferromagnetic-fluid shift begins to that in which it ends.